The present invention relates to a fluid bearing construction for an electric motor, and in particular to a fluid bearing construction that includes a thrust plate having pressure compensation ports.
Rotary spindle machines such as electric motors for disk drives may employ fluid bearings in lieu of traditional ball bearings for supporting the rotary member relative to the non-rotary member. An example of a fluid bearing construction is shown in my copending application, Ser. No. 08/226,310 entitled FLUID BEARING WITH ASYMMETRICAL GROOVE PATTERN which is assigned to the same assignee herein. Other fluid bearings are shown in the U.S. Pat. Nos. 5,112,142 to Titcomb, Shinohara 4,445,793 and Anderson 4,726,693.
Such devices typically include a shaft having at least one axial thrust plate and a hub, which may be a rotary hub, having a sleeve portion generally enclosing the shaft and thrust plate, thus forming a journal bearing with bearing fluid disposed therein. The bearing fluid will form capillary seals at one or more ends of the shaft that are exposed to ambient air pressure.
The problem with such constructions is that under certain conditions the capillary seal may break down and fluid may leak from the bearing. Disruption of the seal may be caused by shock or vibration. Under certain conditions the rotating portion of the bearing may be displaced along the axis of the shaft. In the normal course of events, lubricant flows around the end of the thrust plate from the side with decreasing clearance to the side with increasing clearance. If, however, because of sudden shock or vibration, the lubricant flow around the thrust plate is impeded, fluid will be pushed toward one end of the shaft or the other, possibly breaking down the surface tension which holds the seal in place.
Leakage may also occur when there are inaccuracies in the fabrication of the patterned grooves used by the thrust plate's upper and lower surfaces to maintain a desired net pressure gradient. The object of the grooves is to create a high pressure region in the middle of each thrust plate surface and to create ambient pressure zones at the inner diameter of the thrust plate, adjacent the shaft, and at the outer diameter in the gap between the radially outermost edge of the thrust plate and the sleeve. This type of pressure distribution ordinarily results in no displacement of bearing fluid, that is, the static pressures will equalize. However, fabrication inaccuracies do occur, as does tilt in the bearing, or any other physical phenomena, and these factors can alter the pressure balance in the bearing fluid resulting in flow across the bearing. The flow of bearing fluid can overcome the surface tension seal at either end of the bearing and cause the fluid to leak. The situation is particularly acute at the thrust plate end where pressure imbalances between the upper and lower surfaces of the thrust plate may create a net flow which is near the capillary seal at the upper surface of the thrust plate.
What is needed, therefore, is a construction for a fluid bearing which is capable of equalizing the fluid pressures around the thrust plate, particularly at the inner diameters. If these pressures are equalized, there will be no net flow out of the thrust plate region and, therefore, no breakdown of the capillary seal.